Salt rejecting membranes for reverse osmosis applications have been prepared for more than 40 years. For example, U.S. Pat. No. 3,133,137 generally describes the preparation of cellulose based membranes for reverse osmosis by an immersion precipitation process. In this process, cellulose esters are dissolved in water soluble solvents then spread in a thin layer on a fabric backing. The fabric is immersed in water, causing the cellulosic plastic to precipitate from solution, forming a membrane bonded to the fabric. A drawing of a typical 1960's era casting machine is shown in FIG. 1. The structure of the membrane is termed “asymmetric” because the plastic solidifies with a thin, dense, surface layer (less than 10 microns in thickness) while the remainder of the cellulosic material solidifies to form a spongy microporous material. This structure is well suited for reverse osmosis (RO) because the flux through a membrane is inversely dependent on the thickness of the dense surface layer, while the microporous layer has little effect on the flux in RO applications. Fabric supports are required for mechanical strength since both the porous sub-layer and thin surface layer have little resistance to tearing. In reverse osmosis however, the fabric supports cause only minor resistance to water flux.
The past 45 years of development of highly selective membranes has been focused primarily on the reverse osmosis process. Reverse osmosis is a pressure driven process. That is, once the osmotic pressure of the solution is overcome, the primary resistance to water flux through the membrane is hydrodynamic. This means the friction of water flowing through the pores of the membrane is the primary flow resistance. In RO membranes the flow resistance is almost entirely in the thin dense surface layer, (“rejection layer”) and the microporous support layer and fabric backing have a minor or insignificant resistance.
Forward osmosis, by contrast, is a diffusion driven process instead of pressure driven process, so the factors affecting water flux are dramatically different than reverse osmosis. Because of this, a high performance forward osmosis membrane requires a dramatically different structure than RO membranes. The present invention was made to address this need for forward osmosis membranes.